So here's a pic of the lower part of a 6KD6 tube that tested dead as a doornail. Heater lights up just fine, but no plate current will pass through this tube.
A close up of the failure makes it a bit clearer.
We call this the "blown fuse" since it resembles the element in a glass fuse.
But the blown conductor with the gap in it is the connection from the tube's cathode to pin number 2. I suspect that excessive RF current at this spot inside the tube is the culprit. This fault seems to associate with folks who ran an amplifier way out of tune for too long. Mistuning a tube-type amplifier can cause abnormally-high RF voltages that cause tuning controls to arc over, or abnormally-high RF currents at quirky locations in the circuit.
Quirky, like inside the tube, maybe? Can't prove this, don't have a way to tap in and measure what went on, but a quick look for this fault inside a tube could avoid some amount of head scratching, if an amplifier just won't do anything at all.
73
A close up of the failure makes it a bit clearer.
We call this the "blown fuse" since it resembles the element in a glass fuse.
But the blown conductor with the gap in it is the connection from the tube's cathode to pin number 2. I suspect that excessive RF current at this spot inside the tube is the culprit. This fault seems to associate with folks who ran an amplifier way out of tune for too long. Mistuning a tube-type amplifier can cause abnormally-high RF voltages that cause tuning controls to arc over, or abnormally-high RF currents at quirky locations in the circuit.
Quirky, like inside the tube, maybe? Can't prove this, don't have a way to tap in and measure what went on, but a quick look for this fault inside a tube could avoid some amount of head scratching, if an amplifier just won't do anything at all.
73
